Semiconductors are manufactured in an integral fashion on a wafer of semiconductor material. Such wafers are commonly, but not exclusively single crystals of silicon, gallium arsenide, indium phosphide, gallium nitride, germanium etc. The economy of manufacture is created by producing hundreds or thousands of the same semiconductor device or circuit, in mass, on a single wafer at one time. The devices are organized on the wafer in rows and columns.
After the semiconductors are manufactured on the wafer, the devices must be separated from each other (singulated) so they can be used individually. This processing is called wafer dicing. Wafer dicing is performed by cutting or scribing and breaking along the separation areas (streets) between the rows and columns. Some of the apparatus for cutting are rotary saws and laser burning. Some of the apparatus for scribing and breaking are sharp tool and laser scribing.
Sharp tool scribing is the oldest technique and been practiced since the Semiconductor Industry began in the 1960s before that when glass was invented. A scribing method is described in U.S. Pat. No. 4,095,344, dated Jun. 29, 1978, and entitled “Scribe Tool and Mount Therefore”, to James W. Loomis, one of the present inventors. An improved method of dicing scribed wafers was shown in U.S. Pat. No. 5,458,269, to James W. Loomis, dated Oct. 17, 1995. Each of the forgoing patents is incorporated in its entirety herein by reference.
There have been many promising innovations in semiconductor separation methods since the Loomis '344 patent was issued, and even since the '269 patent was issued, particularly in the area of laser cutting technology. However, the scribing and breaking method of wafer singulation continues to have several advantages over the sawing and cutting methods. In particular, the scribing and breaking of wafers does not create appreciable particle and dust contamination. Thin semiconductor wafers are exquisitely sensitive to contamination by small charged particles, and both abrasive sawing and laser cutting techniques generate a considerable volume of particles and dust that tend to redeposit on the wafer surface. Cleaning of such particles is challenging because the particles adhere to the wafer surface with remarkable tenacity through the van der Waals force, electrostatic forces, and capillary action. The mechanical forces required to overcome the attaching forces and to remove the contaminant particles are often more than sufficient to damage the devices by compromising wire bonds or generating short circuits.
Accordingly, methods were devised to protect the wafer from dust and particle contamination. One method employs a thin protective layer of photo resist, which is peeled from the wafer after singulation through an etching process. Another employs rinsing the wafer as it is sawn in a wet sawing process. Yet another entails covering the wafer with a thin sheet of DI water during sawing. All are expensive and time consuming and the latter two produce a slurry which itself may contaminate the wafer, thus producing a poor product yield.
Sharp point scribing and breaking of thin semiconductor wafers does not generate appreciable contaminant dust and small particles. It is relatively fast and inexpensive, and it reduces the method steps employed in the fabrication processes. As an older and well-established method, it has also reached a stage of considerable refinement. Thus, the method is still preferred by many manufacturers.
However mature it may be in relation to other singulation methods, sharp point scribing has not transcended the need for improvement. One feature of the scribe and break method that limits its efficiency is that the cutting edge of diamond tipped scribes quickly dull through use. After even a single pass over the surface of a wafer, the cutting edge begins to dull and degrade and its ability to scribe the surface sufficiently for damage-free breaking diminishes. In the case of diamond tipped scribe tools, it is a common practice to routinely change the angle of the tool manually after a predetermined number of passes depending on the nature of the substrate, the depth of the scribing, and the quality of the cutting edge. The durability under any set of circumstances can now be fairly accurately predicted from numerous prior microscopic observations of scribe points in use.
Because diamond tips have multiple scribe edges formed in the lapping and polishing process, changing the angle very slightly can bring a new portion of a cutting edge or an altogether new edge into engagement with the wafer, thereby ensuring optimum cutting efficiency. Thus, there has arisen a need to automatically move and control cutting edge engagement with wafer surface during the singulation process.
Scribing and breaking is a phenomenon not well understood. A proper scribe line is a ductile deformation created in the scribed surface. A ductile formed scribe will break without creating dust and cracking. Brittle materials will behave in a ductile fashion when scribed with a microscopically sharp point. The ductile deformation freezes immediately as the deforming point passes a spot x/t. Upon freezing very high stress is created lateral to the scribe line. If the point is sharp and is replicated in the frozen deformation a vertical crack will form under the scribe line. This crack, under the frozen deformation, is a controlled fracture. Applying tensile strain to the crack causes the crack to grow through the wafer.
Creating this deformation and the resulting scribe line causes very high wear on the sharp point. Because of this high wear, the material of choice for sharp points is diamond. Diamond is the hardest material in nature, has a low coefficient of friction and has a thermal conductivity greater than copper. If the scribe point is formed in the proper crystalline structure of the diamond, the point will have optimum wear. Loomis Industries has developed manufacturing techniques that provide scribe tools that are durable and consistent. This consistency in manufacture gives consistency in scribing and consistency in wear life. Consistency is the essence of all manufacturing; it is extremely important for scribe dicing. Scribing must be 100% consistent if break yield is to be high. Knowing how long a point will last is critical so the point can be removed/changed before end of life. If the longevity of a point is ninety meters, the point must be replaced before that. When should a point be replaced? Usually replacement is at the completion of a wafer. A wafer that is 150 mm diameter with dice that are 1×1 mm requires 34 meters of scribing. If point replacement is effected prior to 90 meters and the point is changed after wafer completion, then the point must be replaced at 68 meters (only having produced two wafers). However, if the scribe point lasts 350 meters, then ten wafers can be scribed.
Increased point life creates the following advantages: (1) reduced tool cost (perhaps as little as one fifth the costs for conventional scribing methods; (2) greater than 99 percent yield; (3) greater machine productivity; and (4) improved product quality.